This application claims the priority of PCT International Application No. PCT/EP99/01146, filed Feb. 23, 1999 and German patent document No. 198 10 468.5, filed Mar. 11, 1998, the disclosures of which is expressly incorporated by reference herein.
The present invention relates to a circuit arrangement for supplying electrical energy to a power supply which has a fuel cell and an accumulator device which delivers electrical energy during the starting procedure.
Such a circuit arrangement is disclosed in Journal of the Electrochemical Society, vol. 118, No. 5, May 1971, pages 812-817; K. V. Kordesch: Hydrogen-Air/Lead Battery Hybrid System for Vehicle Propulsion. According to this publication, a fuel cell and an accumulator arrangement are connected to one another by a diode whose polarity prevents a flow of current from the accumulator arrangement to the fuel cell, to prevent outgassing while the fuel cell is idle. To simplify the starting procedure, the voltage of the accumulator arrangement can be applied to the fuel cell for starting purposes, by shorting the diode via a resistor and a switch connected in series therewith. A comparatively small current then flows and changes the individual cells in the fuel cell to the correct direction of polarity.
In another known circuit arrangement, disclosed in German patent document DE 197 37 406 A1, the fuel cell and the accumulator arrangement have a permanent, direct-electrical connection between them, irrespective of the operating state of the fuel cell. It is thus possible for outgassing to occur in this arrangement while the fuel cell is idle. In this case, energy is supplied to a motor and to any other loads as a result of these loads being connected to the fuel cell and to the accumulator arrangement by means of a DC/DC converter.
Another circuit arrangement is disclosed in German patent document DE 44 31 747 A1, in which a fuel cell is connected to other units of a power supply via a DC/DC converter. One of the other units is a battery. A diode is provided, whose direction of polarity prevents a flow of current from the battery to the fuel cell via the DC/DC converter.
Relatively recently, fuel cells have been used as mobile energy generation systems, requiring one or more auxiliary units for their operation. If these auxiliary units are operated electrically, they are supplied with electrical energy by the fuel cell during operation. Usually, a mobile energy generation system having fuel cells, such as a vehicle powered by fuel cells, is not operated continuously; rather, operating pauses occur, and the energy generation system needs to be restarted after such operating pause. Since the energy for driving the auxiliary units is not yet being provided by the fuel cell at the instant of starting, each auxiliary unit requires one or more additional starter units which, if driven electrically, are supplied with electrical energy from a battery.
The object of the present invention is to simplify the starting procedure for a fuel cell.
This and other objects and advantages are achieved by the circuit arrangement according to the present invention, in which electrical energy is fed into the power supply by the accumulator arrangement via at least one DC/DC converter during the starting procedure. In this manner, energy can advantageously be supplied to the auxiliary units during starting via the accumulator arrangement, thereby eliminating the need for additional starter units. Thus, the design is simplified and the weight and volume requirement for the fuel cell system is reduced (which is of particular advantage in the mobile application area). The auxiliary units are put into operation by means of the accumulator arrangement at the beginning of the starting procedure. The DC/DC converter(s) can be used to replace one or more starter units.
In one embodiment of the invention, during operation of the fuel cell (rated operation), the DC/DC converter is connected such that the accumulator arrangement is supplied with electrical energy from the power supply.
Hence, a two-way DC/DC converter is advantageously involved in this case. Advantageously, the on-board charger, which is required anyway for charging the accumulator arrangement, can be used for the purpose of charging the accumulator arrangement. Equivalent components such as transformers, power MOSFETs, capacitors etc. can advantageously be used both for charging the accumulator arrangement and for the starting procedure. During rated operation of the fuel cell, the accumulator arrangement is thus recharged by means of the at least one DC/DC converter.
In another embodiment of the invention, the DC/DC converter automatically distinguishes between rated operation and the starting mode, and changes over accordingly. This simplifies operation of the fuel cell by automatically setting the DC/DC converter to the correct state.
In still another embodiment, the DC/DC converter can be changed over between the starting mode and rated operation by means of an external signal, which may be output by a control unit, for example. When a starting procedure is carried out, the DC/DC converter can be accordingly switched such that electrical energy is fed into the power supply by the accumulator arrangement via the DC/DC converter. In rated operation of the fuel cell, the DC/DC converter can be accordingly controlled such that electrical energy is transferred to the accumulator arrangement from the power supply, which charges the accumulator arrangement.
According to another feature of the invention, the DC/DC converter delivers a signal when it is ready for the starting mode or rated operation, so that the starting procedure can advantageously begin at the correct instant.
Finally, in still another embodiment of the invention, the DC/DC converter charges the intermediate circuit capacitors of the auxiliary units during startup, which thus dispenses with the need for a separate recharging circuit.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.